Swapping an MCB on a Hager Consumer Unit

Hi,

i just oust moved into a new build and am upgrading the showers and need to swap two 40amp MCBs for 50amps.

I don't really want to get the supplier in to remove my main supply fuse. What's the safest way to proceed?

Kind regards, Mike.

Welcome to the forum, you say a new build? I would think that the 40amp MCBs are the correct size for the cable and installation method adopted. MCB's are selected to protect the cable from overload, just increasing the MCB without verifying the cable calculations could lead of overheating and cable damage. The safest way to proceed is to contact a local electrician to come and verify if it is safe to upgrade to 50A. This sort of work is often outside of the scope for DIY projects.

Doc H.  

Thanks Doc, the existing showers are 9.5kW which is why the MCB is 40amps.

The new showers are 10.8kW hence the need for a higher rated MCB.

The existing cable is 10mm so no problem there.

My query is simply: "what is the safest method without removing the supply fuse?"

Kind regards, Mike.

1. Confirm the current cable is suitable for 50 amps. 

2. Isolate the circuit in question using safe isolation procedures. 

3. Remove the existing MCB. 

4. Install the new MCB. 

5. Carry out dead tests to BS7671. 

6. Carry out live tests to BS7671. 

7. Close up all covers securely. 

8. Complete. 

Mike Floutier said:

My query is simply: "what is the safest method without removing the supply fuse?"

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Do you not think that the main switch is an adequate form of isolation? I assume that there is one.

I find the suppliers ratings are a little "optomistic"

I installed a 10KW shower just last week, and the actual current that it draws I measured at 37 amps so it is fine to stay on a 40A mcb, so you could well find your 40A mcb is still okay.

But you DO need to verify what size cable is in use before you do anything.

A lot can go wrong if you get shower wiring wrong so this is not really a job for an inexperienced DIYer.

Mike Floutier said:

Thanks Doc, the existing showers are 9.5kW which is why the MCB is 40amps.

The new showers are 10.8kW hence the need for a higher rated MCB.

The existing cable is 10mm so no problem there.

My query is simply: "what is the safest method without removing the supply fuse?"

Kind regards, Mike.

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From your first line you appear to be missing the point that an MCB is not chosen because of the load. The MCB is chosen for the cable. It is the cable that is selected for the load taking into account the de-rating factors due to its method of installation. If the existing cable is 10.0mm and its a new build with insulation all over the place, there could be a problem if you opt for 50A MCB. Table F6 in the on-site-guide recommends that 10.0mm flat Twin & Earth has a capacity ranging from 32A to 64A dependent upon the installation method. It is quite possible you may need a 16.0mm cable to safely supply a 10.8kw shower. That is of course if you genuinely want to adopt the safest methods and comply with the guidance in the wiring regulations. You will also need to redo the maximum demand calculations for the installation if you are fitting two 10.8kw showers. Your query is simple and the simple answers have been provided. Just swapping the MCB without the relevant calculations and tests is not the safest method. Nor is it advisable to do as a DIY Project.    

Doc H.

Many thanks guys, I really appreciate you taking the time to go through this with me; there is certainly more to think about than I realised.

 The installation and certification was only done around four weeks ago so I have written to the installer to ask his advice.

 Assuming his response is encouraging I'm guessing that it's safe to isolate the MCBs I'm replacing by tripping their controlling RCD and then simply replacing the individual showers' MCBs (if considered necessary).

as mentioned, the MCB is to protect the cable. now whilst a 10mm cable may appear adequate for the additional load, depending on how its installed it may not be. cables have to be increase in some cases, so it may well be that in other circumstances, a 6mm could have been used, but was undersized for that particular install so 10 was used

Thanks Andy, it's a single run of around 10 metres direct from the Consumer Unit to the shower; nothing else on the circuit.

what is the installation method of the 10mm T&E?

You could also find that you isolation switches aren't rated for 50A,,, so these might also need replacing

New showers seem to pull less when new. I see a lot of overrated showers tripping MCBs that have been in for years working fine. The customer just looks at you as if you are stupid when you tell them the installation is sub standard and needs to be rewired or a new lower powered shower and just say "well it has worked for X years why can't you fix it"

"why do I need this arseedee thing? " The fuse has never blown"

:lol:

I blame a general dumbing down of the population in general...

Nobody can even wire a plug these days. 

doormatt26 said:

New showers seem to pull less when new. I see a lot of overrated showers tripping MCBs that have been in for years working fine. The customer just looks at you as if you are stupid when you tell them the installation is sub standard and needs to be rewired or a new lower powered shower and just say "well it has worked for X years why can't you fix it"

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I think you are looking it upside down, Its not the shower drawing more load as it ages , its the MCB being weakened from years of minor overload and going out easier

Possibly or greater friction and less efficient as parts age/scale up. Probably a bit of both. 

Hi Guys,

I’m the OP; I simply left the existing 40A MCB in place and for 6 or 7 years all was fine.

Gradually though, after that time, the MCB started tripping. Very occasionally at first but gradually more frequently, over the last year or so.

It’s come to a head as we’ve got guests, In a couple of weeks, who will be using it, possibly when we’re not around

I’ve noticed it only trips with my wife, not me. She uses the “Airboost” feature, which employs a pump, whereas I don’t. I guessing this makes the difference.

My plan is to replace the 40A MCB with a new one of the same rating.

Just really posting for completeness’ sake

Kind regards, Mike.

MikeFloutier said:

Hi Guys,

I’m the OP; I simply left the existing 40A MCB in place and for 6 or 7 years all was fine.

Gradually though, after that time, the MCB started tripping. Very occasionally at first but gradually more frequently, over the last year or so.

It’s come to a head as we’ve got guests, In a couple of weeks, who will be using it, possibly when we’re not around

I’ve noticed it only trips with my wife, not me. She uses the “Airboost” feature, which employs a pump, whereas I don’t. I guessing this makes the difference.

My plan is to replace the 40A MCB with a new one of the same rating.

Just really posting for completeness’ sake

Kind regards, Mike.

Click to expand...

changing MCB' to higher rating for the guests whilst you're not around? guessing its the inlaws you're trying to kill & a nice insurance claim when the house burns down due to overloaded cables?

Andy™ said:

changing MCB' to higher rating for the guests whilst you're not around? guessing its the inlaws you're trying to kill & a nice insurance claim when the house burns down due to overloaded cables?

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He says he’s replacing with the same rating Andy.

roys said:

He says he’s replacing with the same rating Andy.

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missed that... just went by the original 40 to 50